Method for acquiring measurements and scanner implementing said method

ABSTRACT

In order to solve an analogue/digital conversion dynamic swing problem encountered with detectors, especially solid-state detectors, of a tomodensitometer, there is provision to operate converters at a greater frequency, that is to say more often. In doing this, the converters used need not have as large a conversion dynamic swing. It is shown that instead of a 20-bit conversion, one can make do with a 14-bit conversion. The various conversions are added together to construct the signal. The measurement is thereafter switched, by way of improvement, as a function of the level of the signal received. This measurement is performed according to one mode of use or another, in which modes this acceleration of the rate of analogue/digital conversion is or is not effected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for acquiring measurements with anX-ray tomodensitometer, and a tomodensitometer for implementing theprocess. These apparatuses generally use a single source of X-rays and amulti-element detector opposite the source. The source and detectorassembly can revolve and or move in translation relative to the body ofa patient who is placed between the source and the detector.

2. Discussion of the Background

In general, the X-ray detector includes a multiplicity of individualdetection elements juxtaposed so as to cover the whole of a circular arcilluminated by the X-ray source. There are nevertheless detectors whichare not curved. The most well known detectors are gas-based detectorsand solid-state detectors. The signals output by the detector elementsare gathered periodically, digitized and transmitted to a computer intandem with the rotation, and or with the translation of the apparatus,so as to be processed by digital computation in order to reconstruct aninternal image of a slice of the body exposed to the X-rays.

For greater image resolution, it is desirable to have a large number ofdetection elements juxtaposed along a circular arc, or more generallyalong a line situated in a plane substantially perpendicular to the axisof rotation translation. Moreover, the detector can be constructed inthe form of several detection assemblies juxtaposed in a directionparallel to the axis of rotation translation. In this case, theindividual detection elements can be arranged as several strips of twoor more elements. These elements are aligned in a direction parallel tothe axis of rotation, the strips being juxtaposed along the circulararc. This makes it possible in particular to simultaneously explore twoor more very fine juxtaposed slices of the body examined, since thesource, by revolving, simultaneously exposes two or more adjacent seriesof detectors of small dimension. Such an apparatus is for exampledescribed in the document U.S. Pat. No. 5,592,523.

By way of example, a tomodensitometer can use as detection elements upto several thousand photodiodes, of the polarized type or of thephotovoltaic type, individually covered by a scintillator crystal, andthis number could even increase to several tens of thousands. Theexample which will be studied here will include around 25,000 detectionelements. This results in difficulties with regard to gathering theelectrical signals emanating from these numerous detection elements. Theaim of the invention is to propose a mode of operation allowing the bestpossible solution of the technical constraints resulting from thepresence of this large number of detection elements.

Among these constraints, there are:

the necessity to read the signals from all the detection elements over avery short duration, for example of the order of 0.5 milliseconds. Thisis because a tomodensitometer revolves continuously at a speed, forexample conventional, of two revolutions per second. In a preferredexample, one wishes to perform around 1000 views of the body for eachrevolution. A view corresponds to a given duration in the course ofwhich the body is exposed to X-ray radiation, in a continuous or pulsedmanner. The image of the view revealed on the detector corresponds tothe phenomenon of radiological absorption which occurs for the durationof the view. One distinguishes the image from the view itself (portrayeddirectly in projection mode by the detector), and the reconstructedimage of a body section in which this view participates in thereconstruction calculations. For the duration of a view, thetomodensitometer is regarded as occupying a fixed position relative tothe body when processing the image. Of course, this is not true sincethe tomodensitometer is moving continuously. A view is nevertheless thusassociated with an angle of incidence, a position: the tagging of themean angle at which a principal axis of X-ray radiation irradiates thebody. If the duration of measurement, the duration of view, were greaterthe final resolution of the image would not be acceptable: it would thenbe necessary either to take fewer views, or rotate the tomodensitometerless quickly;

the necessity to read signals over a very large dynamic swing, forexample of the order of 20 bits, since the dark parts of the image yieldan extremely weak signal relative to the lighter parts, and this weaksignal must however be measured with some resolution (a few bits). Thus,in order to measure both 1,000,000 X-ray photons received in nominalmode, or 4 X-ray photons received at minimum, a measurement dynamicswing of 20 bits would be necessary. However, if the overall dynamicswing of the image thus represents 20 bits, the useful local dynamicswing is less demanding. This is because this useful local dynamic swingis equal to the signal-to-noise ratio. In actual fact, thesignal-to-noise ratio is equal to around 1000 maximum. It is in factequal to the square root of the number of X-ray photons received owingto the quantum phenomenon of absorption of X-rays in the body and theirconversion in a scintillator. This means that out of the 20 bitsmeasured, only 14 bits are significant. The digitizing of themeasurements over such a dynamic swing nevertheless involves theadoption of circuits which are overdimensioned in terms of performanceand occupy too much room in an integrated circuit;

in a solid-state detector, the necessity to process quantities of chargewhich may be fairly high when the detector receives large fluxes ofX-ray radiation. Nominal charges of the order of 100 picocoulombs, pC,per photo diode are in fact encountered. Such charges require largestorage areas. This, however, poses size problems with regard toachieving the required capacities for storing these charges, given thelow operating voltages of these integrated circuits. By way of example,the detectors can be diodes of the reverse-bias type, a capacitor beingfashioned at the terminals of a diode owing to its reverse bias. Apreamplifier can be connected successively to each of these diodes so asto recharge the capacitor with the charges which it lost under theaction of the light received by the diode. The quantity of chargereinjected by the preamplifier forms the measurement signal. Preferably,for linearity reasons, non-biased diodes, of the photovoltaic type, willbe used. They produce a DC current proportional to the illuminationwhich they receive. The measurement of the X-ray radiation is effectedtherein via a link from this photovoltaic diode to an integrator, andvia the integration over a small duration of this current signal. Ateach new duration the integrator is previously reset to zero. In thislatter case the integrator is the facility for storing the charges to bemeasured whereas in the previous case it is the reverse-fashionedcapacitor.

SUMMARY OF THE INVENTION

To allow the construction of an apparatus complying with theseconstraints without involving a prohibitive manufacturing cost, thepresent invention proposes a process for obtaining tomographic images,preferably using at least one array of elementary detectors, a chargestorage element associated with each detector, and a circuit, amultiplexer, associated with the array of storage elements so as toperiodically instruct the connecting of the various detectors to theirstorage elements. The successive analogue signals output by themultiplexer are transformed into digital signals downstream by ananalogue/digital converter.

The principle of the invention then consists, in the course of a view,in splitting up the signal integration time so as to reduce the quantityof charge to be measured. Indeed, under these conditions the detectorfacility used need no longer be capable of accumulating large quantitiesof charge. Moreover, the analogue/digital converter no longer needs sucha large dynamic swing. It can be shown that one thus goes from 20 bitsto 14 bits. Because samplings and multiple quantizations are theneffected during a view, it is then of course necessary to execute asummation of the quantization results so as to produce a signalcorresponding to the view, for each detector.

However, the sought-after result is attained even so, namely:

the capacity required for the storage elements is reduced by a factor n(n being a measurements acceleration factor),

the frequency of the analogue/digital converter is increased by thefactor n,

the dynamic swing of this analogue/digital converter is, overall,reduced in a factor lying between n and root n.

The subject of the invention is therefore a process for acquiringmeasurements with a tomodensitometer comprising the following steps:

a body is irradiated, during a given view, with X-ray radiation,

an analogue signal is measured for this view, in each detector of anarray of detectors, this signal representing the effect of theabsorption of the X-ray radiation in the body at the location of each ofthese detectors,

each analogue detector signal is sampled and converted into a digitaldetector signal, characterized in that

the analogue signal from each detector is sampled with n repetitions inthe course of the view, and

the n converted signals from each detector are added together toconstruct the digital view signal of each detector.

Its subject is also a tomodensitometer furnished with a device foracquiring measurements comprising

an X-ray tube for irradiating a body with X-ray radiation duringsuccessive views,

an array of detectors for measuring an analogue signal representing theeffect of the absorption of the X-ray radiation in the body at thelocation of each of these detectors during a view,

an analogue/digital converter for sampling and converting each analoguedetector signal into a digital detector signal,

a sequencer for driving the array of detectors and the converter at therate of the views, characterized in that it includes

means in the sequencer for driving the array of detectors and theconverter at a rate n times greater than the rate of the views and,

an adder for adding together the n converted signals from each detectorso as to construct the digital view signal of each detector.

Another problem to be solved with such tomodensitometers is related tothe number of their detectors, which as stated hereinabove may be verylarge. The assembly of control circuits of all these detectors thenconstitutes a very voluminous system to be constructed, even whenemploying the most modern miniaturization techniques. It is not inreality easy to go from manufacturing a detector with 700 detectionelements to a detector with 25,000 detection elements.

To solve this other problem, according to another characteristic of theinvention, groupings of detectors are constructed and common processingis applied to all the detectors of these groups. The invention starts infact from the following principles which it has highlighted. Firstly, ina projection-mode image, the change in contrast is never abrupt, even ifthe dynamic swing of measurement over the entire image is itself large.This is because the organs of the body of a patient eitherinterpenetrate or are viewed in projection through other organs.Therefore, there is always a transition zone between the images of theseorgans. This transition zone is relatively large on the scale of thesize of the detectors. For this transition zone, the contrast may thenbe regarded as not changing excessively.

Secondly, and by way of adjunct, because of the slow rotation of thetomodensitometer, one may regard the absorption phenomenon measured in adetector, at the moment of a view, as being the same phenomenon (withthe same dynamic swing) as the phenomenon which occurs in a neighbouringdetector at a following view.

Stated otherwise, in the invention, it has then been considered that itwas already possible to group the detectors of a region. The effect ofthis grouping is to subject the detectors of a group to one and the samemode of measurement. In practice, these considerations have led to theconstruction of groups of detectors for which the measurement dynamicswing may be regarded as lying within the same range. According to theinvention, each group of detectors is then allocated a measurementrange. This leads to the simplifying of the electronic detectioncircuits. The range is determined by ensuring that the greatestmeasurement performed for a detector of a group lies within the smallestpossible measurement range assigned to this group. By way ofimprovement, the determination is performed during a view, and theassignment is carried out at the following view.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the description whichfollows and on examining the figures which accompany it. The latter aregiven merely by way of wholly non-limiting indication of the invention.The figures show:

FIG. 1: the diagrammatic representation of the essential means of atomodensitometer;

FIG. 2: a representation of the modification of the detection circuit ofa tomodensitometer so as to enable it to implement the process of theinvention;

FIGS. 3a to 3 g: time charts of signals implemented respectively in thestate of the art and in the invention, and corresponding to the process;

FIG. 4: a chart showing various modes of use of the process of theinvention as a function of the value of the signals detected;

FIG. 5: a diagrammatic representation of the embodiment of a detectionmodule used in the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the essential elements of a tomodensitometer. Thistomodensitometer is useable in the framework of the invention. Thistomodensitometer includes a source 1 producing X-ray radiation 2 whichirradiates a body 3 interposed between the source 1 and a detector 4.The detector 4 can furthermore include an auxiliary detector 5 situatedoutside the X-ray field masked by the body 3. The auxiliary detector 5can serve to normalize the measurements performed. The tomodensitometerrevolves about an axis of rotation whose trace 6 is visible. Thedetector 4 is a detector including a multiplicity of detection elements7.

The detector 4 includes a layer 71 of scintillator elements superimposedon a layer 72 of detection elements proper. The scintillator elements ofthe layer 71 perform a conversion of the X-rays into light rays to whichthe photodetector elements of the subjacent detection layer 72 aresensitive.

According to the sensitivity allowed at present, for one X-ray photonreceived in the layer 71, around 1000 light photons are produced by ascintillator crystal element. The scintillator crystal elements areseparated from one another by transition walls between one crystal andanother.

FIG. 2 shows an exemplary embodiment of the detection elements of thedetector 4. This detector 4 includes an assembly of modules 8. Themodules 8 are matrices of detection elements. The modules 8 are placedside by side in the direction of the length of the detector 4. In oneexample, the length 9 of a module is of the order of 20 mm. In the sameexample, there may be between 30 and 50 modules aligned in the directionof the length of the detector 4. This results in a detector length ofbetween 60 cm and 1 metre. In this same example, the width 10 of themodule 8, in the part thereof useful for detection, is of the order of64 mm. This therefore makes it possible to acquire sections situatedwithin a thickness of the order of 40 mm inside the interposed body 3.The purpose of the numerical examples given hereinbelow is merely tosimplify the explanation and they cannot lead to a limiting of the scopeof the protection obtained by the invention. In this example, themodules include an arrangement of 32 rows, stacked one above the otherin the direction of the width 10, and 16 columns placed side by side inthe direction of the length 9 of the detector 4. Therefore, the module 8includes 512 elementary detectors. To fix matters, it will be presumedmoreover that the tomodensitometer revolves at a speed of tworevolutions per second and that one wishes to perform 1000 views overeach revolution. The duration of a view is therefore 500 microseconds.In the course of each of the views, in the course of each of these 500microseconds, it is appropriate to measure for all the modules, and forall the elementary detectors in each module, the illumination signaldetected.

The bottom of FIG. 2 shows the architecture of the embodiment of thedetectors of a module 8. Each module of detectors preferably consists offour subgroups 11 to 14 of elementary detectors and of associatedprocessing circuits. The subgroup 11 thus includes 128 detectors denoted15 to 17. In this example, these detectors are diodes of thephotovoltaic type. The subgroups include for example detectors situatedin 8 adjacent columns out of 16 and in 16 rows out of 32. As a variant,two subgroups per module are constructed.

The photodiodes 15 to 17 are detection elements installed in the layer72 of photodetector elements of the detector 4. They receive luminousradiation corresponding to the X-ray radiation received at the locationof the scintillator crystal which surmounts them. These diodes arelinked by their two terminals, on the one hand, in common to earth, andon the other hand, in an individualized manner, each to the input of anamplifier 18 to 20 respectively. The amplifiers 18 to 20 are operationaltype amplifiers. In one example, they can consist of a simpletransistor. The amplifiers 18 to 20 are mounted as integrators by way ofcapacitors 21 to 23 respectively which loop their outputs back to theirinputs. The outputs of the amplifiers 18 to 20 are furthermore linked tostorage capacitors 24 to 26 respectively. The link between the outputsof the amplifiers 18 to 20 and the capacitors 24 to 26 is constructed byway of switches 27 to 29 controlled by a signal S1.

When the switches 27 to 29 are closed, the amplifiers 18 to 20 chargethe capacitors 24 to 26 instantaneously. The charge injected into thecapacitors is dependent on the voltage which the integrators 18 to 20have reached on termination of an integration duration (corresponding toa view, hence corresponding to 500 microseconds in practice).

When the voltage available at the outputs of the amplifiers 18 to 20 hasbeen transmitted to the capacitors 24 to 26, the amplifiers 18 to 20 arereinitialized by a signal S2 applied to a gang of switches 30 to 32respectively, shunted to the capacitors 21 to 23. The reinitializationis instantaneous. On termination of the signal S2, the diodes 15 to 17recommence injecting current into the amplifiers 18 to 20.

A multiplexer 33 makes it possible to link, each in turn, the storagecapacitors 24 to 26 to an analogue/digital converter 34 which, in apreferred example of the invention, is a 14-bit analogue/digitalconverter.

In the state of the art represented by FIGS. 3a to 3 c which show thesynchronizations of the signals S1 to S3, the signal delivered by theconverter 34 was a signal relating to a view. It was also distinct foreach of the detectors 15 to 17. FIG. 3c shows in particular that asignal S3 which includes 8×16=128 pulses made it possible, in the courseof a view, to sample, each in turn, and to digitize the signalscontained in the 128 capacitors 24 to 26. With the four subgroups 11 to14 one therefore had the 4×128=512 measurements corresponding to amodule, for each view. The duration of the view is here the durationwhich separates the pulses 35 and 36 of the signal S1, or 37 and 38 ofthe signal S2 in FIGS. 3a and 3 b.

It will be observed that, in the module including 512 elementarydetectors, there are 4 converters 34 per module 8. There is one for eachof the subgroups 11 to 14. Given the number of modules, 50, there are200 converters 34 in the device of the invention. Moreover, theseconverters are simple.

Given the large dynamic swing of the signal to be measured, and thelikewise large number of converters 34, it was appropriate, according tothe invention, to advocate converters 34 of smaller size (14-bit) ratherthan converters according to the state of the art (20-bit). In theinvention, to solve this problem, it was decided to increase thefrequency with which the converters 34 sample then digitize the contentsof the capacitors 24 to 26.

Correspondingly, the frequency of transfer of the output voltages fromthe amplifiers 18 to 20 to the capacitors 24 to 26 is increased in thesame way. This is shown in FIGS. 3d to 3 f corresponding to FIGS. 3a to3 c respectively. It is seen that in these groups of figures, theduration D of a view remains the same. In one example, it is still 500microseconds. On the other hand, in the invention, the integration,sampling and quantization are executed n times during this period. In apreferred example, n=8. The significance of the signal S3 of FIG. 3f isthat the converter 34 delivers the 128 results, n times more frequentlythan in the framework of FIG. 3c.

In the invention, furthermore, as and when the results are delivered bythe converter 34 in respect of a detector, they are added in an adder 39to results corresponding to the same detector and delivered by this sameconverter, but at a previous quantization. For this purpose, a firstmultiplexer 40 which also runs at the rate of the signal S3, taps offfrom a memory 41, at an address 42 (changing at the rate of the signalS3), the result of a previous quantization which had been stored there.This result is stored in a buffer memory 43. The buffer memory 43 istied up with the adder 39. At the due moment, the adder 39correspondingly adds the content of the memory 43 to the resultdelivered by the converter 34. The output of the adder 39 is linked to asecond multiplexer 44 whose role is to store, again at the address 42,the result of the addition of the old content of the address 42 with thequantization result delivered by the converter 34.

FIG. 3g shows a supplementary signal: the signal S4. The signal S4 isthe signal synchronous to the signal S1 or to the signal S2 of the stateof the art, FIGS. 3a and 3 b. The signal S4 controls a third multiplexer45 which rapidly extracts from the memory 41 the 128 data which werestored there. The memory 41 includes memory cells preferably of 20 bits.The reading of the memory 41 by the multiplexer 45 must be fast. This isbecause it must occur during the first of the n quantizations carriedout by the analogue/digital converter 34 in the course of the view.

If need be, the multiplexer 45 is combined with the multiplexer 40, thesignal being available on the output 46 only once every n times.

FIG. 4 shows various modes of use of the tomodensitometer of theinvention and of its detection system. The abscissa gives the number ofX-ray photons incident over the duration of a view (500 microseconds) onan elementary detector of the detector 4. The ordinate shows the outputsignal expected after digitization. The scales of the co-ordinate axesare logarithmic so as to take into account a 20-bit large overalldynamic swing. The straight line 47 shows the (normal) processingperformed by the detection chain with a one-for-one conversion rate. Thestraight line 48 also shows the change in the signal-to-noise ratioresulting from the quantum detection by the scintillator crystals. Inparticular, for nominal illumination, the signal-to-noise ratio is equalto 1000 (square root of 1 million).

Depicted on the chart of FIG. 4 are three domains entitled Mode 1, Mode2 and Mode 3 respectively and corresponding to different spans of X-rayilluminations. For Mode 1, the signal detected corresponds to chargeslying between 12.5 pC and 100 pC. According to the invention, forsignals corresponding to this mode, one uses an acceleration of the rateof tapping off of the output voltages from the amplifiers 18, 19 and 20as well as the corresponding quantization by the converter 34.Specifically, in this case, the converters 34 will operate at the rateof the signal S3 of FIG. 3f. In the case where n=8, the result stored inthe cell 42 of the memory 41 will equal, on termination of the view, asignal coded on 17 bits (14+3) positioned in the high-order bits. Thisis because the addition of eight signals coded on 14 bits leads to aresult on 17 bits. The adder 39 is therefore a 17-bit adder.

According to the improvement to the invention, a signal corresponding toeach elementary detector of a subgroup 11 of elementary detectors ismeasured in a comparator 49 linked to the output 46 of the multiplexer45. If the signal from at least one of these elementary detectors isabove, equivalently, 12.5 pC, accelerated acquisition is used for allthe elementary detectors of this subgroup. On the other hand, if thesignal from all the detectors of a subgroup is below 12.5 pC, onedecides no longer to employ the improvement to the invention. For thispurpose, the comparator 49 delivers a signal 50 whose role is totransform the signal S1 visible in FIG. 3d into a signal S1 visible inFIG. 3a. In practice, the frequencies of the signals S1, S2 and S3 arethen divided by n. A clock 51, playing a sequencer role, thereforedelivers, as a function of the signal 50, signals S1, S2, S3 which mayor may not be accelerated. It is easy, with a cyclic counter driven by avery fast clock, to investigate one bit of the signal delivered by thiscounter and to construct the pulses S1 to S3 with the state of this bit.For an eightfold acceleration, it is sufficient to investigate onelower-order bit, to shift by three units.

Preferably, the acceleration is no longer effected when the signaldetected is weak and should the analogue/digital converter'sdigitization noise be greater than the X-ray quantization noise due tothe scintillator crystal.

It has been observed moreover that the gradient of the absorption varieslittle in the image. In practice, the gradient is below 50 or 100. Thissignifies that the signal detected on the detectors of a module 8 is nottoo different from the signal detected on the detectors of an adjacentmodule. This has led to the groups of detectors being constructedaccording to the invention.

Furthermore, given the slow rotation of the tomodensitometer, the zonesof the body which are viewed by adjacent detectors in the module 8, inthe course of a view, are viewed almost by the same detectors of themodule 8 at the following view. Thus one is able to predict on one viewwhat will be the signal on a following view, since locally the signalwill moreover vary little, the contrast having no abrupt changes. Thisthen makes it possible, by way of improvement, when making a measurementfor a view, to decide to apply the signal 50, not in the course of theview, but for a following view. Stated otherwise, the signal 50 whichcauses the toggling from Mode 1 to Mode 2, or vice versa, is applied tothe clock 51 only after the pulse of the signal S4 which marks the endof the current view.

In certain cases, the signal detected is much weaker than an equivalentat 1.6 pC. In this case, in order to use the converter 34 to the maximumof its range, the signal originating from the storage elements 24 to 26is amplified beforehand with an amplifier 52, before conversion. Thisamplification is preferably performed if all the detector signals of agroup are below a threshold. For example, the amplification will be in aratio 8. Stated otherwise, with such an amplification the dynamic swingof the converter 34 is fully used. However, owing to the amplificationcarried out, it is then appropriate to shift, in the cells 42 of thememory 41, the result towards the low-order bits. If the amplificationfactor equals 8, the result has to be shifted by 3 bits in the low-orderdirection. In reality, the amplification factor will not be 8.Consequently, the result of the conversion will be divided by the actualamplification factor. This will be carried out by a circuit interposedbetween the converter 34 and the adder 39.

It will be noted that the thresholds of 12.5 pC and 1.6 pC arearbitrary, although fairly well suited to the problem.

The signal from the comparator 49 which controls the amplifier 52 ismoreover stored in a selection circuit 521. The circuit 521 is then tiedup with the multiplexer 44 so as to bring about, at the moment ofrecording in memory 41, a recording which may or may not be shifted by 3bits towards the low-order bits.

Specifically, one will thus have obtained a dynamic swing of 14+3+3=20bits by only ever using an analogue/digital converter capable of a14-bit dynamic swing.

If need be, the amplification by the amplifier 52 and the accelerationof the rate can be combined.

FIG. 5 shows a practical exemplary embodiment of a module in the casewhere the detector includes photodiodes of photovoltaic type. FIG. 5shows a module 8 including 512 elementary detectors distributed into 32rows and 16 columns. FIG. 5 shows the 4 subgroups 11, 12, 13 and 14,each including 128 photodetectors. In one example, these photodetectorsforming the layer 72 are made on a silicon substrate carried by aceramic support which will moreover carry the control circuits.Metallizations such as 53 are each linked by connections (notrepresented) to an individualized photodetector. The other terminal ofthese photodetectors is linked to earth, a metallization (notrepresented) subjacent to the substrate and common to all.

In one example, the photodetectors correspond to an area of 0.6 mm times1.2 mm, smaller than the dimensions of 1.25 mm times 2 mm givenpreviously. There is therefore a neutralized frame around each detectingzone 55. The purpose of this neutralized frame is to electricallyinsulate the photodetectors from one another and furthermore to adapt tothe size of the scintillator crystals deposited on a photodetector.There is not actually any drawback in acting in this manner since thequantum detection phenomenon occurs in a scintillator crystal of thelayer 71 which surmounts the detector 55 and the frame 54. The photodiode 15 is for its part sensitive to illuminous radiation containing,approximately, 1000 times more photons. In this case, the useful areasof the diodes are smaller than the corresponding area presented by ascintillator crystal which surmounts them.

What is claimed is:
 1. Process for acquiring measurements with atomodensitometer which includes a source of X-rays intended to irradiatea body, during a given view in which: an analogue signal is measured forthis view, in each detector of an array of detectors, this signalrepresenting the effect of the absorption of the X-ray radiation in thebody at the location of each of these detectors, each analogue detectorsignal is sampled and converted into a digital detector signal,characterized in that the analogue signal from each detector is sampledwith n repetitions in the course of the view, and the n convertedsignals from each detector are added together to construct the digitalview signal of each detector.
 2. Process according to claim 1,characterized in that groups of detectors are constructed the signalfrom at least one detector of a group is compared with a threshold, andthe signals from the detectors of the group are sampled with nrepetitions if this signal from at least one detector of the group isabove this threshold.
 3. Process according to claim 1, characterized inthat groups of detectors are constructed, the signals from all thedetectors of a group are compared with a threshold, and the signals fromthe detectors of the group are amplified before conversion if each ofthese signals is below a threshold.
 4. Process according to claim 2characterized in that analogue signals from detectors which correspondto a view subsequent to that during which the comparison is made aresampled with n repetitions or are amplified.
 5. Process according toclaims 1, characterized in that the detectors are photovoltaic diodesand in that in order to make measurements there is integrated in anintegrator, in the guise of detector signal, a current signal producedby each photovoltaic diode over a duration n times smaller than theduration of a view before sampling this detector signal, and theintegrator is reset to zero before a new integration of this current. 6.Tomodensitometer furnished with a device for acquiring measurementscomprising an X-ray tube for irradiating a body with X-ray radiationduring successive views, an array of detectors for measuring an analoguesignal representing the effect of the absorption of the X-ray radiationin the body at the location of each of these detectors during a view, ananalogue/digital converter for sampling and converting each analoguedetector signal into a digital detector signal, a sequencer for drivingthe array of detectors and the converter at the rate of the views,characterized in that it includes means in the sequencer for driving thearray of detectors and the converter at a rate n times greater than therate of the views and, an adder for adding together the n convertedsignals from each detector so as to construct the digital view signal ofeach detector.
 7. Tomodensitometer according to claim 6, characterizedin that it includes groups of detectors, a comparator assigned to agroup so as to compare a signal from at least one detector of a groupwith a threshold, and a circuit for controlling the driving at a rate ntimes greater if this signal from at least one detector of the group isabove this threshold.
 8. Tomodensitometer according to claim 6,characterized in that it includes groups of detectors, a comparatorassigned to a group for comparing the signals from all the detectors ofa group with a threshold, and an amplifier for amplifying beforeconversion the signals from the detectors of the group of these signalsare below a threshold.
 9. Tomodensitometer according to claim 7,characterized in that a group detectors includes two or four detectorsubgroups mounted with their control circuits all together on the samesupport, for example a ceramic support.
 10. Tomodensitometer accordingto claim 6, characterized in that the detectors are photovoltaic diodeseach associated with a controlled integrator.
 11. Tomodensitometeraccording to claim 6, characterized in that the detectors include photodiodes whose detection area is less than the detection area of ascintillator crystal which surmounts them.